Linear light modulators form images by a rapid, repeated sequence in which each single line of the image is separately formed and is directed to a screen or other display surface by reflection, or other type of redirection, from a scanning element. Types of linear light modulators that operate in this manner include devices such as grating light valve (GLV) designs, offered by Silicon Light Machines and described in U.S. Pat. No. 6,215,579 (Bloom et al.), and elsewhere. Display systems based on GLV devices are disclosed, for example, in U.S. Pat. No. 5,982,553 (Bloom et al.).
An improved type of linear light modulator is the grating electro-mechanical system (GEMS) device, as disclosed in commonly-assigned U.S. Pat. No. 6,307,663 (Kowarz), and elsewhere. Display systems based on a linear array of conformal GEMS devices are described in commonly-assigned U.S. Pat. No. 6,411,425, and U.S. Pat. No. 6,476,848 (both to Kowarz et al.). Further detailed description of GEMS device architecture and operation is given in a number of commonly-assigned U.S. patents and published applications, including U.S. Pat. No. 6,663,788 (Kowarz et al.), and U.S. Pat. No. 6,802,613 (Agostinelli et al.). In GEMS devices, light is modulated by diffraction. On a GEMS chip, the linear array of conformal electromechanical ribbon elements, formed on a single substrate, provides one or more diffracted orders of light to form each line of pixels for line-scanned projection display.
GLV and GEMS color display system architectures generally employ three separate color paths, red, green, and blue (RGB), each color path provided with a linear array of electromechanical grating devices. Each linear array of electromechanical grating devices, when actuated, modulates its component red, green, or blue laser light to form a single line of the image at a time. The resulting modulated lines of light for each color are then combined onto the same output axis to provide a full-color image that is then scanned to the display screen.
In general, linear light modulator arrays are advantaged over their area array spatial light modulator (SLM) counterparts by virtue of higher resolution, reduced cost, and simplified illumination optics. GLV and GEMS devices are actuable to operate at fast switching speeds and are able to modulate laser light. GLV and GEMS devices have advantages for high resolution, high native bit depth, variable aspect ratio, and freedom from motion artifacts, when compared with other types of spatial light modulators. However, there are some inherent limitations for display solutions that use these devices. The galvanometrically actuated scanning mirror that is conventionally used to scan modulated light across the display surface scans by rotating over a short angular range to form a single 2-D (two-dimensional) frame of the image, then must be reset, rotating back into position for the next scan. During this reset time, no image content can be projected using the standard scanning scheme. Over about 15-25% of the time, the mirror is rotating back into position for the next scan. This reduces the available light output and limits the light efficiencies that can be obtained. Due to mirror reset time and acceleration and deceleration times of the scanning mirror, the effective duty cycle for providing modulated light with such systems, the so-called “pixel on” time, is no more than about 72-82%.
One response to the need for improved efficiency has been to change mirror cycle timing and to project image content during rotation of the scanning mirror in each direction. This strategy is described in U.S. Pat. No. 7,053,930 (Webb et al.). This approach projects modulated light over an additional amount of time, gaining between 5 and 10% improvement in efficiency over the earlier timing cycle, but requires multiple refreshes of the same image frame during projection. Bidirectional scanning for 2-D imaging also sacrifices image resolution somewhat in practice, particularly for displays having high resolution.
Stereoscopic projection is one area of particular interest for cinema projection overall. Conventional configurations for stereo projection include configurations that use two projectors, one for the left eye and the other for the right eye. This basic model has been applied with earlier film-based systems, as well as with digital projection equipment, from vendors such as Barco Corporation. Although such two-projector designs have successfully shown the feasibility and enhanced imaging capabilities afforded by stereoscopic imaging systems, these systems are expensive, require precision alignment to each other, and impose some additional requirements on theater design and layout. Because stereoscopic display methods usually provide alternate left- and right-eye images, separated in time, they typically require more than twice the nominal light output of 2-D displays to achieve the same brightness. Stereoscopic solutions have been implemented using a single projector, but generally at the cost of reduced light output. Conventional solutions such as doubling the number of light sources or doubling the number of light modulators are feasible, but are also expensive and impractical.
The conventional modulation scheme that has been used for 2-D imaging does not easily adapt to the requirements of stereoscopic or 3-D (three-dimensional) imaging. Simply doubling frame refresh rates from 60 Hz to 120 Hz and alternating left- and right-eye image sub-frames does not appear to be a desirable solution. Higher frame rates place demands on scanning devices, such as a galvanometric scanner, that can be difficult to meet. The higher frame rate necessary for stereoscopic imaging using conventional techniques increases the bandwidth requirement for projector electronics and reduces the minimum pixel drive pulse width for pulse-width modulated light valve arrays.
In the face of these difficulties, linear light modulators seem unlikely contenders for the stereoscopic imaging market. The excessive cost of providing multiple projector devices, registered to each other for independently providing left- and right-eye images, and the complex data paths, alignment, and timing that would be required to coordinate and project left- and right-eye modulated light make dual-projector solutions unattractive and well out of the price range of lower-cost equipment. Other conventional approaches to stereoscopic display using a single projector have required considerable complexity and, due at least in part to disappointing brightness in the displayed output, may not yield pleasing results in the final image that is displayed.
Thus, there is need for stereoscopic display methods that can take advantage of the particular strengths of GEMS, GLV, and other linear light modulators for forming stereoscopic images.